In molding elastomeric materials, i.e., those materials that become cross-linked during the molding process in such fashion as to retain their final shape indefinitely, it is known that there are several parameters which must be taken into account to ensure the release of entrapped air and gases from the molded part and thereby prevent voids in the part. It is also known that the problem of voids in parts molded from elastomeric material is particularly acute since these materials cannot be remolded. Unlike thermoplastic materials, a defective part molded from an elastomeric material cannot be reground and molded again.
The major parameters affecting the molding of elastomeric materials are mold temperature, injection pressure (if injection molding process is being used), clamp pressure and cure time. It is known that the adjustment of these parameters may help eliminate the voids in the molded part. It is also known that the adjustment of these parameters affects other properties of the molding material. Thus, it is possible that by adjusting the parameters to eliminate the voids in the molded part, other problems may be created such as gas blisters, under-filled parts, and adherence of parts to the mold.
The problem of entrapped air or gas in the molding material can become more acute with the use of different filler materials such as magnetic oxides or the like. Oftentimes a different filler material will dramatically change the molding parameters associated with the elastomeric material. Thus, adjusting the above-mentioned parameters to facilitate the elimination of voids in the parts may have an adverse effect in relation to other problems that can occur in the molding process.
Although molds can usually be vented to allow entrapped air or gas to escape, some types of mold design represent compromises between venting and charge weight which is the amount of material loaded into the mold. Such mold designs often produce parts with an unacceptable degree of air entrapment when used with certain elastomers, notably those with unusually good permeability characteristics, such as butyl rubber, chlorosulfonated polyethylene, etc. As compression mold designs rarely allow a charge weight greater than 110 percent of the weight of the finished part, it is often impossible to adjust molding parameters to compensate for limited venting.
It was found in molding an elastomeric material with a magnetic oxide filler for a magnetic drum for telephone announcement systems that an entrapped air or gas problem was apparent in the molded part. This created special problems because the drum was subject to substantial additional (and expensive) processing, and the drum had to be discarded if there were voids therein. The voids normally were too deep to be machined out during subsequent processing. Thus, unless this problem was eliminated, there would be considerable economic waste. Conventional adjustment of the molding parameters was found inadequate for preventing the occurrence of voids in the material.
It is an object of this invention to prepare a charge that will eliminate the entrapped air and gas from a part molded from an elastomeric material containing a magnetic oxide or a like filler material and thereby facilitate the production of quality parts by substantially eliminating the occurrence of voids in the molded part.